The content of nitrogen compounds in the Wel river
in the light of the Nitrates Directive
Maciej Operacz, Andrzej Kentzer, Łukasz Nowaczyk
Department of Hydrobiology, Institute, of Ecology and Environmental Protection, Nicolaus Copernicus University, Gagarina 9, 87-100 Torun, Poland
e-mail: operacz@umk.pl
Abstract. In this paper, the results of the research are presented on the content and transformation of nitrogen compounds in the Wel
River (the largest tributary of the Drwęca River).The catchment area of the studied river is predominantly agriculturally developed. Along the river course, 6 sample collection sites were selected, situated in places with different land development of the drainage basin. The average concentration of total nitrogen in the water of the Wel River amounted to 1.807 mg/dm3, mineral nitrogen – 0.557
mg/dm3 and organic nitrogen – 1.250 mg/dm3.The organic form of nitrogen dominated at all sites (the average content for the whole
river amounted to 69%).Nitrate nitrogen dominated in the pool of mineral nitrogen. The lowest concentrations of nitrates were being recorded at lakeland sites, whereas the highest – at two terminal sites situated within the lower reaches of the river, where the drain-age basin has a typically agricultural character.
Key words: Nitrates Directive, Wel River, flow-through lakes, agricultural catchment.
1. Introduction
The content of nitrogen is one of the main indicators used for the assessment of the quality of surface waters, whereas the concentration of mineral nitrogen compounds (ammo-nium and nitrate ions) is an important criterion for deter-mining the usefulness of water for consumption (Hatfield & Follett 2008).Nitrogen gets into the surface waters from the drainage basin due to mineralization of organic matter, together with wastewaters, with atmospheric precipitation, from the atmosphere (diffusion), and also as a result of fixation of atmospheric nitrogen by microorganisms, such as blue-green algae and bacteria. a number of studies and researches revealed, however, that agriculture is the main source of contamination of surface waters with nitrogen compounds. Fertilizer components unused in the agricul-ture can penetrate into underground and surface waters, causing their contamination. This type of pollution is de-scribed as nonpoint source pollution and may affect up
to 70% of the agriculturally exploited area of the country (Sapek 1996).
In the surface waters, nitrogen occurs in two forms: organic and mineral one. Mineral forms of nitrogen are readily soluble and usually get into the surface waters from the drainage basin (mainly nonpoint sources). The organic form of nitrogen usually inflows together with soil particles (erosion) and untreated wastewaters. During the growing season, usually the organic form dominates in the surface waters, while during the winter season – the mineral form. Whereas, in the conditions of good oxygenation, nitrates have the main contribution in the mineral form (Kajak 1998).As an important biogenic element, it significantly influences the processes of eutrophication taking place in water bodies.The dynamics of nitrogen compounds is very complex due to the fact that this element occurs in many forms (Dodds 2002).
Due to a significant increase in the contamination of surface waters with nitrogen compounds, recorded in the last decades, in 1991 the European Union introduced the
During the whole study period, considerable variabil-ity in the concentrations of total nitrogen in the Wel River was observed.At the first three sites, the concentration of total nitrogen basically has not been changing. Only the percentage contribution of mineral and organic forms has been changing. Transport of nitrogen is disturbed by the presence of open lakes. It is obvious that open lakes are the place of intensive nitrogen transformation.In the wa-ter flowing out of the lakes, a considerable increase was recorded in the content of organic nitrogen contained in the suspended matter and in planktonic organisms. Organic nitrogen was a dominant form in the total nitrogen.
This confirms the results obtained by Gołdyn (2000). The Cybina River after flowing through Lake Swarzędz contained much higher amounts of total nitrogen.This kind of situation, however, was taking place only in the summer months, during which the primary production proceeds on a larger scale as compared to winter months.Whereas Hill-bricht-Ilkowska and Kostrzewska-Szlakowska (1996) did not find any statistically significant differences between lakes and the Krutynia River regarding the retention of total nitrogen. Also, it was not possible to describe ex-plicitly the transport of nitrogen in the Babant River. The accumulation of this element was recorded in both stud-ied lakes (Kufel 1996). Three open lakes of the headwater catchment of the Radunia River (Nowiński & Lange 2004) were undoubtedly of depositional significance for nitrogen. Lake Stężyckie, located at the highest altitude in the cas-cade system, functioned as a „sorptive complex” in relation to the remaining reservoirs, located at the lower altitude. The content of total nitrogen was decreasing as the water flows through three lakes located one after the other, from the value of 3.5 mg/dm3, through 1.0 mg/dm3, up to the value of 0.2 mg/dm3 in the lake situated at the lowest alti-tude.Different results, from these presented in this paper, were obtained by Lossow et al. (2006).They studied three open lakes, which were characterized by positive reten-tion for nitrogen: 40.5%, 11.1% and 23.8% respectively for particular lakes.The authors report that the main form of nitrogen (similarly like in the Wel River) was organic nitrogen. Most probably this dissimilarity results from dif-ferent hydrological conditions of open lakes.
Average concentration of total nitrogen at the level of 1.41 mg/dm3 was obtained by Lossow et al. (2006), who investigated rivers of north-eastern Poland – Marózka and Łyna, similar to the Wel River.When investigating the Ba-tices;
– designation of areas particularly vulnerable to contami-nation of waters with nitrates;
– defining obligatory principles for the management of fertilizers and principles of agricultural science within the zones of particular sensitivity.
2. Research methods
Researches related to transformation of nitrogen com-pounds in the Wel River were conducted at monthly inter-vals from March to December 2008.Water samples were collected at six sites distributed along the entire length of the river, from its source to the mouth (Fig. 1).The Wel River is the largest tributary of the Drwęca River (a drink-ing water source for Toruń) with the length of ca. 110 km and the catchment area of ca. 800 km².The average flow within the estuary section is ca. 5m³/s (Główczyński et al. 2010). The catchment area of the Wel River is an area of outstanding nature and landscape values.Along a consid-erable length, the river flows through the Wel Landscape Park, which was created in order to protect the river.
Concentrations of mineral forms of nitrogen (nitrate ni-trogen N–NO3 and ammonia nitrogen N–NH4) were deter-mined in the collected water, using the standard methods of hydrochemical analysis (Hermanowicz et al. 1999). Or-ganic nitrogen was determined with the Kjeldahl method. Concentrations of mineral nitrogen were obtained from calculations according to the formula:
Nmin. = N – NO3 + N – NH4.
Concentrations of organic nitrogen were calculated ac-cording to the formula:
Norg. = Nk. – N – NH4.
Concentration of total nitrogen was calculated accord-ing to the formula:
Ntot. = Nmin. + Norg.
3. Results and discussion
The average value of the total nitrogen concentration for the Wel River along its entire length amounted to 1.807 mg/dm3 during the research period (Fig. 2, 3). It enables to classify the waters of the studied river within purity class I. The average value at particular sites ranged from
Figure 1. Fluvio-lacustrine system of the Wel River together with the drainage basin and sample collection sites: 1 – Dąbrówno, 2 – Szczupliny, 3 – Grabacz, 4 – Koszelewki, 5 – Trzcin, 6 – Bratian
Figure 2. Average concentrations [mg/dm3] of total nitrogen at particular sites (March – December 2008)
0 0,5 1 1,5 2 2,5
Dąbrów no Szczupliny Grabacz Koszelew ki Trzcin Bratian
sampling sites
mg/dm3
.
.
bant River (the Masurian Lakeland), Kufel (1996) obtained lower values of the total nitrogen.The average concentra-tion of Ntot. in this river amounted to 1.11 mg/dm3. In the rivers of the Coastal Region, as well as in the Oder River and the Vistula River studied by Bogdanowicz (2004), the concentration of total nitrogen did not exceed the value of 5 mg/dm3.The lowest value he obtained for the Reda River was 1.9 mg/dm3, whereas the highest for the Rivers Ina and Oder – 4.15 mg/dm3. Whereas Granlund et al. (2005), who studied small Finnish rivers, draining mainly agricultural lands, recorded the average concentration of Ntot., amount-ing to 2.96 mg/dm3.
The average value of mineral nitrogen for the Wel River along its whole length, during the research period, amount-ed to 0.557 mg/dm3 (Fig. 4, 5). The average value at partic-ular sites ranged from 0.244 mg/dm3 at the site Dąbrówno (the source) to 0.911 mg/dm3 at the site Bratnia (the estu-ary) (Fig. 4).During the winter months, higher concentra-tions of mineral nitrogen were being recorded as compared to summer months.At all sites, the percentage contribution of the mineral form was lower as compared with the per-centage contribution of the organic form (Fig. 3). At the terminal, estuary site (Bratian), the content of Nmin. in Ntot. amounted to 45%. At the first site (Dąbrówno), the content of Nmin. in Ntot. was only 15%.
At four, out of six, research sites, nitrate nitrogen was the dominant form of mineral nitrogen (Fig. 5). The av-erage concentration of N–NO3 during the research period amounted to 0.348 mg/dm3. The ammonia form dominated only at two middle sites (Grabacz and Koszelewki).From the obtained results, one can see a considerable increase in the concentration of nitrate nitrogen along the river course.
At the last site, the concentration of N–NO3 was five times higher than at the first site. The increase in the content of this form of mineral nitrogen was brought about by the influence of a few factors. The waters receive the pollu-tion from nearby areas with the surface run-off.Almost 70% of the catchment area of the Wel River is composed of agricultural lands, which are the main source of river water pollution with mineral forms of nitrogen. Tributar-ies, which disgorge into the river, intensively drain the ag-ricultural lands, as well as carry the waters from nearby fish ponds.The Wel River receives also wastewaters from nearby treatment plants and industrial plants. The afore-mentioned factors, as well as the absence of open lakes in the lower reaches of the river bring about the fact that the contribution of mineral nitrogen increased along the river course.According to Bogdanowicz (2004), the high content of nitrates proves the significance of runoff from agricultural lands. Also Hatfield and Follett (2008), as well as Hillbricht-Ilkowska and Kostrzewska-Szlakowska (1996) report that waters flowing down the agricultural ar-eas are characterized by high content of this nitrogen form. Slightly higher concentrations of nitrates during the winter months could be accounted for by the reduced primary pro-duction, and at the same time smaller demands for N–NO3. Eighty five per cent of the rivers studied by Arheimer and Liden (2000) had significantly lower concentrations of ni-trates during the summer months (May–August).Also the Vilaine River, studied by Moreau et al. (1998), was char-acterized by the highest concentrations of nitrates during the winter months and the lowest ones during the summer months. In lakeland rivers of NE Poland, the average con-centration of N–NO3 amounted to 0.293 mg/dm3 (Zieliński
Figure 3. Contribution of mineral and organic nitrogen in Ntot. at particular sites (March–December 2008)
0 0,5 1
Dąbrów no Szczupliny Grabacz Koszelew ki Trzcin Bratian
sampling sites
mg/dm3
2003).Whereas Bogdanowicz (2004), in his researches on the Vistula and Oder Rivers, as well as on rivers of the Coastal Region, obtained the average value of 1.535 mg N–NO3/dm3.
The lowest value of the nitrate nitrogen concentration in the presented studies was recorded at the sites down-stream of the open lakes.This seems to be obvious due to intense assimilation of this form of nitrogen by
phyto-plankton in the process of primary production.Gołdyn et al. (2005) recorded nearly 30% reduction in the concentra-tion of N–NO3 at sites downstream from the flow-through lakes.Furthermore, reduction of mineral nitrogen in lakes is determined by the process of denitrification. Kajak (1998) reports that denitrification is one of the main proc-esses contributing to a decrease in the nitrogen content in water bodies.He estimates that up to 3 mg nitrogen could
Figure 4. Average concentrations [mg/dm3] of mineral nitrogen at particular sites (March – December 2008)
Figure 5. Contribution of ammonia nitrogen and nitrate nitrogen in the pool of mineral nitrogen at particular sites (March–December 2008) 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1
Dąbrów no Szczupliny Grabacz Koszelew ki Trzcin Bratian
sampling sites mg/dm3 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1
Dąbrów no Szczupliny Grabacz Koszelew ki Trzcin Bratian
sampling sites mg/dm3 N-NH4 N-NO3 . . . . . . . . . . . . . . . . . .
20 µmol N/m²/h within the headwater section to 659 µmol N/m²/h at the river mouth.
No nitrate-dilution effect has been observed along the river course.Allan (1998) reports that in rivers, in which the supply with this biogenic element is relatively constant, the increasing water volume will dilute a given substance. In the Wel River, where point sources of pollution are rare and nonpoint sources have a considerable contribution in water pollution of the river, the dilution effect has not been observed together with the increasing mileage of the river. a similar situation was observed by Lossow et al. (2006) who studied rivers similar to Wel, i.e. Marózka and Łyna. Concentrations of nitrates depend mainly on the water flow rate and they were increasing along the river course. Dif-ferent results were obtained by Hillbricht-Ilkowska and Kostrzewska-Szlakowska (1996), who investigated the Krutynia River. In the Krutynia River, the concentration of the nitrate form was decreasing along the river course.The dilution effect was observed for nitrates received by the river together with its tributary within the initial reaches.
Only at two sites in the middle reaches of the river (Grabacz, Koszelewki), the ammonia form has a higher contribution in the pool of mineral nitrogen (Fig. 5),a con-siderable contribution of the ammonia ion most probably could be attributed to the influence exerted by process-es of decomposition of organic matter flowing out of the open lakes (the process of ammonification). Additionally, the water outflowing from the lakes is usually leaner in oxygen, which leads to a decrease in the nitrification rate. Variable results were obtained by Gołdyn and Kowalcze-wska – Madura (2005) in their studies on the Cybina River. And thus, after the river flows out of Lake Iwno, a drop in the concentration of the ammonia form was recorded, from the value of 0.924 mg/dm3 to the value of 0.650 mg/dm3. Whereas along the further reaches of the river, after flow-ing out of Lake Swarzędz, an increase in the concentra-tion of N–NH4 was recorded, from the value of 0.711 mg/ dm3 up to the value of 0.902 mg/dm3.The highest average concentration of ammonia nitrogen was recorded at the site Grabacz. Much higher concentrations of ammonia nitrogen at this site were most probably connected with the presence of fish-breeding ponds in the neighbourhood.Fish ponds contribute locally with very high loads of ammonia due to high density of specimens in a relatively small water vol-ume (low dilution) and their intense feeding (Kajak 1998). Lossow et al. (2006) observed an increase in the
concentra-age concentration of ammonia nitrogen in 58 rivers of NE Poland amounted to 0.211 mg/dm3. The lowest average concentrations of ammonia nitrogen were recorded in the Szkwa River (14.4 μg/dm3), and the highest ones in the Leśna Prawa River (more than 2 mg/dm3) (Zieliński 2003). Bogdanowicz (2004), who studied the Vistula and Oder Rivers, as well as rivers of the Coastal Region recorded the average concentration of N–NH4 equal to 0.247 mg/dm3).
4. Conclusions
1. According to guidelines of the Nitrates Directive (the maximum concentration for Ntot.> 50 mg/dm³ and the maximum concentration for N – NO3 > 11.3 mg/dm³), the water of the studied river is characterized by very low, safe for the environment, content of nitrogen compounds.
2. The content of each of the analysed nitrogen forms (nitrate nitrogen, ammonia nitrogen, mineral, organic and total nitrogen) allow to classify the water of the Wel River from March to December 2008 along its entire length – from the sources till the mouth – within the 1st category of surface water purity.
3. Nitrate nitrogen was the dominant form of mineral nitrogen. In the total nitrogen, the organic form predomi-nates over the mineral one.
4. Flow-through (open) lakes have a significant influ-ence on the transformation of nitrogen compounds in the studied river.At the sites located downstream of the open lakes, the content of mineral nitrogen (particularly the ni-trate form) was evidently decreasing. Whereas, the content of organic nitrogen was increasing.
5. The lower reaches of the river were characterized by much higher concentrations of mineral nitrogen as com-pared with the upper and middle reaches.
Acknowledgements
The authors wish to extend their gratitude to Mr Krzysztof Główczyński, the Director of the Wel Landscape Park, for his comprehensive assistance during the field works.
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